ABS and Brake light stuck on.?

Also, I appear to going 30mph all the time. Brakes are working. Last time this hapened I lost my brakes, but this time it's different. Now the common answer to this problem states its an ABS sensor. Well here is my real question. The part for my car (95' Honda Prelude SI) says there is a ABS sensor on each hub on each wheel. All are $85. How do I know which one to replace? My town is notorious for shady mechanics. This problem has rip off written all over it.